In some known power inverters for feeding electric energy from a DC power generator into an AC grid with two-power lines, the power inverter comprises a DC/DC converter for matching the output voltage of the power generator to the grid, and a DC/AC output converter for actually feeding the electric power from the DC power generator into the AC grid.
In such known power inverters the DC/DC converter can be a converter including a high frequency transformer comprising a primary winding and a secondary winding. Such a transformer generally provides for a galvanic isolation of the secondary or output side from the primary or input side of the power inverter. A DC/DC converter including a high frequency transformer further comprises at least one high frequency switched semiconductor power switch that, in its closed state, connects one end of the primary winding of the high frequency transformer to one of the input terminals of the power inverter for providing a current path through the primary winding to the other one of the input terminals. The alternating current through the primary winding may also be provided by any type of inverter bridge connected between the input terminals of the power inverter.
The current through the secondary winding of the high frequency transformer is rectified by a high frequency rectifier typically comprising diodes arranged as a rectifier bridge, and a filter capacitor connected between output lines of the high frequency rectifier.
An interesting sub-class of DC/DC converters used in known power inverters are resonant or quasi-resonant converters, which comprise a resonant circuit. Such a resonant circuit allows for zero voltage and/or zero current switching of the semiconductor power switches providing the alternating current through the primary winding of the high frequency transformer. The resonant circuit may be provided on the primary side or on the secondary side of the high frequency transformer, and it can be a resonant parallel or series circuit.
Bob Mammano and Jeff Putsch: Fix-Frequency, Resonant-Switched Pulse Width Modulation with Phase-Shifted Control (http://server.oersted.dtu.dk/ftp/database/Data_CDs/component_data/Unitrode_seminars/se_m800/slup096.pdf) disclose a resonant-switched DC/DC converter comprising two half bridges connected between DC input terminals, each half bridge comprising two semiconductor switches and a center connected to one respective end of a primary winding of a high frequency transformer. The ends of the secondary winding of the high frequency transformer are each connected to a rectifier diode. One end of a filter circuit comprising a series inductor and an output capacitor is connected to a joint output of both rectifier diodes, and its other end is connected to a center tap of the secondary winding. The semiconductor power switches of each half bridge at the primary side of the high frequency transformer are controlled by complementary high frequency signals of 50% duty cycle. Thus, at any time at least one of the semiconductor switches of each half bridge is closed except for a dead time during which the parallel resonant transition occurs. If the semiconductor switches of the two half bridges connected to the same input terminal are closed at the same time, the primary winding of the high frequency transformer is short-circuited via these two semiconductor power switches. Only if just one of these switches is closed whereas the other is open, a current between the input terminals flows through the primary winding of the high frequency transformer. These on-times of the primary side of the high frequency transformer are defined by a phase-offset or phase-shift between the high frequency signals applied to both half bridges. The length of the on-times is defined with conventional PWM and controls the power delivered to the load. Switching of the semiconductor power switches is done at zero voltage.
Both zero voltage switching and its dual equivalent, zero current switching, provide for very low switching losses. However, in zero current switching, it is not possible to use pulse width modulation as a current shaping means with high efficiency. Only a modulation of the repetition rate of the pulses is available, as the pulse widths are determined by the zero current switching criterion.
The DC/AC converters at the output side of some known power inverters are inverter bridges with high frequency switched semiconductor power switches for forming a desired sine shape of the currents fed into the AC grid. Some other known power inverters, however, comprise a line-commutated converter at their output end, the switching elements of which are only controlled by the voltages of a connected AC grid, and are, thus, only switching at the grid frequency. As a result, these DC/AC converters at the output side are unable to provide a sine shape of the current fed into the AC grid, and any current shaping has to be done upstream of the line-commutated converter.
Some photovoltaic modules show a premature degradation in use if not permanently operated at a negative or positive electric potential with regard to electric ground. Further, operating photovoltaic modules at a defined negative or positive electric potential could be used for ground fault detection. Thus, some efforts are made to provide a voltage offset for the input terminals of a power inverter for feeding electric energy from such photovoltaic generators into an AC grid.
Some known power inverters require particular attention during their startup, as dangerously high currents may flow as long as buffer capacitors are not yet loaded to provide a sufficient counter voltage. On the other hand, electric loads present on buffer capacitors in operation of some known power inverters pose a danger when terminating the operation of known power inverters, even if all active parts of the power inverters have been inactive for some time and even if the power inverter has already been disconnected from the grid for some time.
Some regulations, like those in the US, require a galvanic isolation from the public power grid for any power generator from which electric power is fed into the public power grid.
A power inverter for feeding electric energy from a DC power generator into an AC power grid with two-power lines is known from DE 10 2005 023 290 A1. This power inverter is a bi-directional battery inverter and comprises a high frequency transformer. The high frequency transformer and a resonant capacity connected to the secondary winding of the high frequency transformer form a resonant series circuit. The primary winding of the transformer has a center tap and is connected to the battery via a center point circuitry with semiconductor switches. The resonant series circuit is connected to a rectifier. The rectifier is connected to a boost converter that feeds a DC input voltage link of a DC/AC converter.
DE 10 2005 023 291 A1 discloses a power inverter including a resonant converter. The resonant converter comprises a high frequency transformer that forms a resonant series circuit in combination with a resonant capacitance connected to its primary winding. The secondary winding of the high frequency transformer is connected to a rectifier that is connected to a DC voltage input link of a DC/AC converter.
US 2008/0192510 A1 discloses a power inverter similar to the one known from DE 10 2005 023 291 A1. Here, the primary winding of the high frequency transformer is fed by a photovoltaic generator by means of an inverter full bridge, the center point of each of the half bridges of the inverter full bridge being connected to one end of the primary winding.
U.S. Pat. No. 5,587,892 A discloses a multi-phase power converter with harmonic neutralization, in which capacitors are connected to each end of a primary winding of a high frequency transformer. Each of these capacitors is combined with an inductor in addition to the high frequency transformer to provide a resonant series circuit.
There still is a need for a power inverter particularly suitable for feeding electric energy from small to medium sized photovoltaic modules into an AC power grid, the inverter being available at low cost but nevertheless displaying a high performance, i.e. low power losses at a high level of security.